Figure out how to better fix Interconnect inline signals on MinGW

[mosra]

As of 5d2faa1, there's finally a test case and a (partial) solution for signals emitted across shared object boundaries. This problem started appearing on Linux/macOS after introducing -fvisibility-inlines-hidden in 7ae9031 (part of a fix for #53) and can be fixed on those systems by explicitly exporting the inline symbols as well.

MSVC doesn't have this problem in the first place (the inline function is defined just once in the DLL), but MinGW duplicates it and so far I'm not aware of any solution except for moving the definitions into the *.cpp file (which is extremely verbose). The Ui library does that as of mosra/magnum-extras@97da887 to make it usable, but I hope there's a better solution in the longer term.